Field of the Invention
This invention relates to a laser system and laser having improved transverse coherence and to a method of achieving the improved coherence.
More particularly, it relates to an x-ray laser system and laser having improved transverse coherence which is suitable for use in obtaining improved x-ray holographic images.
X-ray laser systems and lasers have been known in the art since about 1984. X-ray lasers are similar to visible light lasers except that they generate x-rays when properly stimulated, or pumped, instead of conventional beams of light. The total "x-ray" region broadly covers 700 .ANG. down to 0.1 .ANG. wavelength. Traditionally, however, the x-ray range is considered to begin at about 300 .ANG.. The x-ray region is further categorized as "soft" for wavelengths in the .about.2-300 .ANG. range and "hard" for shorter wavelengths. X-ray lasers can be used for a number of purposes, including the making of holographic images.
Laser irradiated exploding foils have recently been used to create soft x-ray lasers. When properly designed and irradiated, such foils give rise to elongated plasmas that have high density, nearly constant temperature and smooth transverse density profiles. Models for laser heated exploded foils, wires or fibers and spheres are described in a number of articles, including J. M. Dawson, Phys. Fluids 7, 981 (1964); W. J. Fader, Phys. Fluids 11, 2200 (1968); J. Dawson et al., Phys. Fluids, 12, 8757 (1969); R. E. Kidder, in Physics of High Energy Density (Academic, New York, 1971), p. 306; A. V. Farnsworth, Phys. Fluids 23, 1496 (1980), Rosen et al., Phys. Rev. Letters, 4,106 (1985), and R. A. London and M. D. Rosen, Phys. Fluids 29, 3813 (1986).
Discussions of the feasibility and ultimate utility of x-ray holography of biological samples are given by Solem et al., "Microholography of Living Organisms," Science, 218,229-2357 (1982), M. Howells, "Fundamentals Limits in X-ray Holography," in X-Ray Microscopy II, D. Sayre et al., Eds, (Springer Verlag, New York, 1988) p. 263, and R. A. London, M. D. Rosen and J. E. Trebes, Appl. Optics, 28, 3397 (1989).
X-ray lasers which are to be used for holography, however, require highly coherent wave output. The degree of longitudinal coherence is generally adequate for holographic applications, but the transverse coherence of prior art lasers is generally inadequate to generate high resolution images. Coherence, as used herein, refers to the existence of a correlation between the phases of two or more waves, such as the reference wave and the object wave in the holographic process.
In a normal optical laser, coherence is achieved by operating the laser in a multi-pass cavity. Because the reflectivity of x-ray mirrors is low, (less than about 50%) and the duration of x-ray laser gain is short (of order 250 psec), x-ray lasers have been operated as single pass or few pass (up to three) devices. It has therefore been difficult to design one with good transverse coherence.
Previous methods suggested for improving the transverse coherence involve using a lasing medium with a small cross-sectional area (see Rosen, Trebes and Matthews, "A Strategy for Achieving Spatially Coherent Output from Laboratory X-ray Lasers," Comments in Plasma Physics and Controlled Fusion, Vol. 10 p. 245, (1987)). The method suggested by Rosen et al., appears to be somewhat difficult to implement since it requires a two-component exploding foil--a lasant material surrounded by a non-lasant material, and furthermore, the lasant needs to be initially very thin (=80 .ANG. in their example).
It would be desirable in the art to provide a laser and laser systems having improved transverse coherence, and a method of achieving improved coherence in an X-ray laser.